There are numerous industrial processes where it is necessary to convert the energy of a pressurized stream of gas into rotational energy, particularly in processes where some reaction is involved which gives off a hot, relatively high pressure gas, such as processes for coal gasification.
In recovering power from such high pressure gases, i.e. letdown, an expansion turbine is frequently used and in some instances, a radial inflow turbine is utilized in which rows of nozzles direct the high pressure gas onto the outside ends of vanes of a rotatable wheel, the motive fluid thereby gives up its pressure energy to the wheel and the fluid is exhausted along the axis of the wheel at some lower pressure.
One difficulty that is encountered in such turbines and in the systems in which they are used, is that some streams of high pressure motive gas contain solid particulate matter which erodes away the material of the turbine blades, eventually leading to their failure and thus expensive downtime during which blade replacement can be accomplished. The solid particulate matter, or particles is inherent in certain industrial and chemical processes and, unless removed, can cause damage to the main letdown turbine or to further rotating equipment downstream of such turbine.
In particular, in several of the processes now under investigation for coal gasification, a stream of high-temperature, high pressure gas is produced which contains a relatively high percentage, i.e. as much as 1-2% of solid particles which are extremely hard and erosive to turbine blade surfaces. Also, the process currently used in refineries to produce lighter fuels, such as gasoline from crude oil known as the fluid catalytic cracking process, also inherently produces a hot, high pressure gas stream containing particles in this case, minute catalyst particles that are carried downstream.
Since it is important for plant efficiency, and in some instances to to very economics of the process itself, to extract some of the power from the then developed hot, high pressure gases, the use of a turbine is desirable. The gas streams, being heavily laden with erosive particles, must somehow be treated to remove particles to prevent erosion of downstream equipment.
In such hot gas streams, conventional filtration techniques have limitations that detract or prevent their use. For example, since such gas streams are generally high temperature gases (on the order of 825.degree. C.) and high pressure, conventional filter and bag houses are not practical. Cyclone separators, on the other hand, may be capable of functioning under higher temperature gases but they cannot separate particles smaller than about 5 microns. Smaller particles are, however, still erosive to blade surfaces.
In either instance, the device or means used introduces some finite pressure drop which is all lost, that is, none of the energy given up during the pressure drop is recovered as useful energy.
Attempts have been made in the turbine prior art to construct a letdown turbine capable of converting the energy in the high pressure gas to useful rotational energy while effecting some particulate separation, as in U.S. Pat. No. 3,944,380, but such prior art turbines are susceptible of erosion in the turbine nozzle block resulting from the hot gas containing particles being centrifuged outwardly and being circulated around the nozzle block with high velocity prior to bleeding to external collection device. The sliding of such particles through small passageways or along the blades still cause erosion.